This invention relates to dishwashing machines, particularly commercial machines which utilize a recirculating wash system of relatively high flow capacity, and which may incorporate some form of conveyor and/or racks to carry soiled articles through the machine. Types of machines to which the invention is applicable are disclosed in U.S. Pat. Nos. 2,073,521; 2,884,935; 3,049,391; 3,067,757; 3,254,698; 3,267,944 and 3,844,299.
These machines are manufactured with various optionally available equipment, one option being the type of heater used to maintain the desired temperature of the recirculated wash liquid. Electric immersion heaters are mountable with heating elements extending into the tank. Steam injectors can also be used to heat the wash liquid by adding steam to it. In either case, the heat is applied to the pool of liquid within the tank of the machine.
In the case of gas-fired heaters, a gas burner is mounted to the underside of the tank to heat the water by conduction through the tank walls or through an immersion tube. The flue for hot products of combustion runs along a wall of the tank and usually a wall of the washing chamber above, in fact it is a practice to use portions of these walls as parts of the flue construction. Thus these tank and chamber walls are subjected to substantial temperature increase, and care must be exercised to avoid thermal cycling of these walls and various joints they form with other structure of the machine. If the pump is turned off, as for a temporary stoppage, while the gas heater continues to run at full output, the change in liquid level in the tank and the lack of liquid splashing over the chamber walls and upper walls of the tank produces quite a different thermal gradient in the wall than under dynamic conditions with the pump running. These changing temperature conditions can lead to thermal fatigue, of these materials which results in cracking of tank walls or joints, and can require major service to the machine.